All about Tubes, Tube Circuits, Tube Gear

Thursday, June 14, 2012

Tube Biasing 101: (Part 1) Biasing a 6922

oh oh, I've seen schematics like this before,
and he's driving the 6922 too hard: taking it over its dissipation-wattage rating.

But others have also pointed out that there is nothing really to gain from that, except shortened tube life.

OK this schematic came from here:

DIY 6922 / E88CC Tube preamp

On that page you'll see his loadline for the 6922:

If you plot the Watt Dissipation line over this, you'll see the problem.

The solution is to alter the loadline (by putting a db-pad or volume pot on the output) and also the quiescent-point (idle-current) by adjusting the bias via the cathode resistor, and possibly the voltage.

The 6922 is a great tube, but it is a low-noise fragile signal tube, not meant for this kind of manhandling.

The Dot which represents the idle-spot should be well below the max-dissipation curve.
The voltage swing, which will meander over the line anyway, should never spend more than half its time outside the boundaries.

Although Philips allows 2 watts dissipation / triode,
The Telefunken sets 1.5 watts as the real limit. Obviously we go with the lower rating.

Also, it is generally recommended that your design stay at 70% of max wattage rating for safety and to allow for variation between tubes and brands.

Here's a better loadline and bias0-point for the 6922.

It sits at 70% of the conservative max dissipation (1 watt).

With this new bias-point you stay in 70% zone, and tube stays out of trouble most of its life.

The load resistor becomes about 12k5, the cathode resistor is 830 ohms, the bias voltage is -5.
The cathode floats about 150 volts above ground.

The tube coasts in its comfy spot, with very little loss of headroom and great gains in tube longevity. 

Tube Charts without Heat-dissipation and max rating boundaries are almost useless for setting tube loads and bias points.

1 comment:

  1. One thing I can't figure out here---what do you mean that the cathode floats 150V above ground? In the original, the cathode was at 90V, because the plate voltage is set at 160. Here, the plate voltage is higher (~175V?) so it seems like the cathode to ground voltage should be less than the original.

    Also, if it's 150V between cathode and ground, isn't this a concern for the potential between the cathode and the heaters?

    Thanks, it's an informative piece.